An Overview of Various Grid Synchronization
Techniques for Single-Phase Grid Integration of
Renewable Distributed Power Generation Systems
Rashmi Ranjan Behera
Prof. AN Thakur
Department of Electrical and
Electronics Engineering
National Institute of Technology Jamshedpur
Jamshedpur, India
Email: rashmiranjan1011@gmail.com
Department of Electrical and
Electronics Engineering
National Institute of Technology Jamshedpur
Jamshedpur, India
Email: anthakur.ee@nitjsr.ac.in
Abstract—Due to the increasing penetration of single-phase
distributed generations into the utility grid the need of an
improved and efficient controller to control the distributed
power generating systems (DGs) is also increasing. Therefore
the grid synchronization techniques plays a major role in order
to maintain the grid requirements in terms of power quality
and the frequency of the grid voltage. The informations of
grid voltage frequency, amplitude and the phase are the basic
requirements for single-phase grid integration. For efficient
estimation of these, various synchronization techniques have been
proposed. The focus of this paper is to review the different
synchronization techniques particularly for grid integration of
single-phase renewable DGs based on Phase Locked Loops (PLL)
and Non-PLL method.
single-phase DG systems can cause adverse effect on reliability, stability and availability of the distributed grid. And in this
scenario adequate synchronization scheme play a major role
in the control of single-phase grid connected systems. Usually
a grid fault exists for a short period of time, so an accurate
and fast synchronization method ensures good performance
of the whole grid connected systems in the grid fault mode
of operation. In some published work Phase Locked Loop
based synchronization method is one of the promising methods
of synchronization with grid. And to eliminate the delay to
synchronize due to PLL the emphasis on PLL less methods is
also increasing [20] - [22].
I. I NTRODUCTION
II. GRID REQUIREMENTS FOR SINGLE - PHASE GRID
The raising concern of rapid depletion of fossil fuels and
the government policies of developed as well as developing
countries on reducing greenhouse gases emissions, fetching a
great interest on more and more distribution generations (DG)
plants. Especially those DGs which are based upon renewable
energy such as Wind turbines and Photo-voltaic(PV) plants.
In recent years, there has been booming installation of singlephase PV systems which are connected to the low-voltage
distribution lines, because of the maturity in PV technologies
and the declining price of the PV panels. As the penetration
and capacities of these DGs is increasing, the need of the
hour is to operate and control them efficiently and effectively
in order to maintain high power quality and dynamic stability.
So pushed by the increasing penetration of renewable energy
systems into the grid, to regulate the interconnected renewable
power generation, there are many grid requirements have been
released [1] - [3]. These grid codes put more rigorous demands
regarding the capabilities of the renewable DG systems to track
the variations in grid-voltage amplitude and frequency and also
to minimize harmonic distortions.
Grid synchronization is one of the major issues for distributed generation systems which are connected to utility
network through Power Electronics systems. Under grid faults
conditions, the disconnection of the large scale grid connected
INTEGRATION
According to standard IEC 61727,the allowable limit for
the current Total Harmonic Distortion is 5%. And also the
limit for odd harmonics, 3rd to 9th is 4% and 11th to 15th
is 2%. The variation limit for the voltage amplitude is 0.85
pu to 1.10 pu, with frequency variation limit ±1 [1] [2].
There is a need to track phase angle, grid voltage amplitude,
frequency for the anti-islanding, over/under voltage protection,
and over/under frequency protection respectively. The grid
connected renewable distributed generation systems should
have accurate, reliable, and fast tracking system to comply
with these requirements in case of abnormalities.
III. SINGLE - PHASE GRID S YNCHRONIZATION TECHNIQUES
For most of the single-phase grid tied power conditioning systems, such as active power filters, dynamic voltage
restores, flexible AC transmission systems (FACTs), uninterrupted power supplies (UPS), the most vital informations
required to connect with grid are phase-angle and frequency
of the utility grid. And to estimate these phase-angle and
frequency of single-phase signals, various methods have been
proposed in previous literatures.
A typical single-phase grid connected renewable DG system
as shown in Fig.1, involves various single-phase loads with un-
as Inducverter based on Induction motor principle has been
published in [20].
A. PLL based techniques
Fig. 1. Single-phase grid tied renewable DG systems with overall control
structure
equal loading of feeders, loads being continuously connected
or disconnected, and loads which are nonlinear,unbalanced and
distort the voltage at the point of common coupling (PCC).
Accurate knowledge of phase and frequency of grid voltage under these conditions is hard to obtain but crucial for
converter operation and control. Various grid synchronization
schemes have been reported to address the grid requirements
and comparative studies of some of them have been carried
out and published in [10], [13] - [22]. Different single-phase
PLL and Non-PLL based techniques has been shown in Fig.2.
The mostly accepted synchronization technique to a time
varying signal shown by Fig.3. Here the phase difference
between the input and output signal is measured by the phase
detector (PD), then it is passed through a loop filter (LF),
which is a low pass filter. The voltage controlled oscillator
(VCO) is driven by the output signal of the loop filter (LF) to
generate the overall output signal, which will follow the input
signal [5].
Fig. 3. Basic PLL structure
1) PLL based on T/4 delay: When the fundamental grid
frequency time period is T, using first-in-first-out (FIFO)
buffer the T/4 transport delay technique can be implemented,
by setting its size to one fourth of the number of samples
contained in one cycle of the fundamental frequency.
Fig. 4. Basic T/4 delay based PLL structure
Fig. 2. Classification of single-phase grid synchronization techniques
Generally a PLL is a closed loop feedback control system,
whose output signal is synchronizes in phase, as well as in
frequency, with the fundamental voltage component of the
grid. An ideal PLL supposed to provide fast and accurate synchronization informations with a high degree of immunity and
insensitivity to disturbances, harmonics, sags/swells, notches
and other distortions. PLL based techniques are quite gaining
the interest and many methods of single-phase PLL based
synchronization techniques has been published in recent literatures. single-phase PLLs are mainly divided into the transport
delay based, Park’s transform based, Hilbert transformationbased, all-pass filter-based, SOGI PLL and robust PLL [12] [19]. In recent publications the focus is on eliminating the
dedicated PLL structure for synchronization process. Because
if there will be any delay in tracking process can cause
instability in the whole system. And so a new technique called
As shown in Fig.4, considering a input signal which is the
component and creating a component by delaying Vi by π/2
i.e T /4 w.r.t. fundamental frequency of the input voltage. And
then transforming these Vα and Vβ , from αβ to dq by using
Park’s transformation i.e eq.(1), it can be used in detecting the
phase error.
vd
cosθ sinθ vα
Vm sin(∆θ)
=
=
(1)
vq
−sinθ cosθ vβ
−Vm sin(∆θ)
This technique is a simple one to extract phase-angle in
single-phase application. If the input voltage signal is purely
sinusoidal waveform at a rated grid frequency then only it
works well [10]. Because the orthogonal signal output will
not be perfect if the grid frequency changes, and this will
rise to errors in synchronization. And also it doesn’t have any
filtering capability, so if there is any harmonic components in
the input voltage, that will disrupt the PLL.
2) PLL based on Hilbert Transform: Hilbert transform
which is also called as ‘quadrature filter’, is a mathematical
tool which have some fascinating features like, it can shift the
phase angle of the input signal by ±π/2, depending on the
sign of their frequency. It affects only the phase-angle without
affecting the amplitude of the signal. For a real signal x(t),
the Hilbert transform (H) is defined as shown in eq. (2), where
P is Cauchy principal value.
Z
P ∞ x(τ )
x̂(t) = H(x) =
(2)
π −∞ t − τ
Fig. 5. Basic structure of PLL based on Hilbert Transform
Hilbert transformer generates a orthogonal signal to the
input signal and then transformed it to dq–axis voltages by
the help of Park’s transformation, which is then used in the
control loop. An ideal Hilbert transformer is not practically
realizable as it violates the causality property of a system [6].
3) PLL based on Inverse Park’s Transform: In this method
to build an orthogonal signal generator, which is supposed to
generate the ‘β’ component, a loop consisting of direct as well
as indirect Park’s transform supported by two low pass filters
is created as shown in Fig.6. But here the condition is the PLL
should be perfectly tuned to the input frequency to make vα to
be in phase with vα0 and in quadrature with vβ0 . And also the
nonlinear inner loop must be fast enough to generate the ‘β’
component [9] [12]. The whole technique can be described by
these set of equations:
"
#
"
#

v
(s)
v
(s)

d
i
vdq (s) =

= Tp 0


vq (s)
vβ (s)







"
#
"
#


0
v
(s)
vα0 (s)
d
vαβ (s) = 0
= Tp−1 0
(3)

v
(s)
v
(s)

q
β




"
#
"
#
"
#



0

v
(s)
(s)
v
v
(s)
d
d

d
ω

= GL (s)
= s+ω
 0
L
vq (s)
vq (s)
vq (s)
Fig. 6. PLL based on Inverse Park’s Transform
4) Enhanced PLL (EPLL): The EPLL is a frequency adaptive nonlinear synchronization approach, based on a simple
adaptive filter (AF), introduced in [19]. Its superiority over
conventional PLL lies in the phase detector (PD) mechanism,
which brings more flexibility and provides informations about
five fundamental signal attributes such as harmonic signal,
corresponding amplitudes, fundamental components, phase
component and frequency component. As shown in Fig. 7, to
get the informations regarding the amplitude of input signal to
extract the phase error, EPLL PD uses an adaptive filter and
a multiplier in such a way that it can increase the information
gathering capability. This technique is very much suitable
in a variable frequency environment because it can provide
very high immunity and insensitivity to harmonics, noise, and
imbalance of the input signal. As it can shift the input signal by
π/2 rad., so it is a suitable alternative in single-phase system
applications [19].
As shown in Fig.7, the objective of the EPLL is to track the
input voltage amplitude Vm and phase-angle θ. The adaptive
filter estimates the amplitude according to the error signal and
locked phase angle. The PD output equation can be written
as:
pd =
Vm
V̂m
Vm
sin(θ − θ̂) +
sin(θ + θ̂) −
sin(2θ̂)
2
2
2
(4)
Considering θ = ωt + ø and θ̂ = ω̂t + ø̂ and when ω̂ = ω
Vm
V̂m
Vm
(ø − ø̂) +
sin(2ωt + ø + ø̂) −
sin(2ω̂t + 2ø̂)
pd ∼
=
2
2
2
(5)
Where sin(ø − ø̂) ∼
= (ø − ø̂)
Fig. 7. Basic structure of Enhanced PLL
5) Robust Single-Phase PLL: By using robust PLL technique we can instantly estimate the phase, frequency, and amplitude information of single-phase signals even in frequency
variation, amplitude sag/swell,phase jump, and harmonic distortion. It consists of two-phase signal generator which produce vα and vβ , a vector rotator, a phase synchronizer, a lowpass filter and a multirate sample-holder has proposed in [16].
The phase synchronizer based on “generalized integral type
PLL method”, which differentiate it from other type of singlephase PLL methods. In case of transients, the differential type
two phase generator is better than the integral type generator.
The vector rotator performs the processing as:
vγ
v
T
= R (θ̂α ) α
(6)
vδ
vβ
Where RT (θ̂α ) is the transpose of R(θ̂α ),which is
cosθ̂α −sinθ̂α
R(θ̂α ) =
sinθ̂α cosθ̂α
(7)
As shown in Fig.8, the final estimated frequency ω̂α is feedback to the two-phase signal generator through the multirate
sample holder, and this reuse of the estimated frequency makes
the estimation system to be auto tuning to varying frequency.
Fig. 8. Basic structure of Robust PLL
6) Second Order Generalized Integrator based PLL (SOGI
PLL): The SOGI-PLL is based on frequency adaptive quadrature signals generation by means of the SOGI-QSG filter [4].
This techniques uses two adaptive filters unlike EPLL, which
uses only one weight adaptive filter. Because by using two
adaptive filters it gives better performance and behaves like a
“sinusoidal integrator” [4]. If ω being the resonant frequency
of SOGI, the transfer function of adaptive filter based on the
Second Order Generalized Integrator is defined as
ωs
(8)
GI(s) = 2
s + ω2
frequency, using Fourier series principle. And assuming the
grid frequency constant and known, the order of the harmonics
can be extracted at the output of this filter. The amplitude and
the phase angle of this frequency component can the calculated
further [4].
2) Inducverters: Some of in recent publications the emphasis on PLL less synchronization process is being increasing.
Because in case of a dedicated synchronization unit specially
based on PLL method, if any large delay occur in PLL, the
the synchronization unit can cause instability, and so it has
a considerable effect on the performance of converter circuit.
Under weak grid conditions, islanded conditions, and due to
large penetration of DGs into grid, this issue becomes more
critical. So using a auto/self synchronization and control of
converters without using a dedicated synchronization unit is
getting increasing interest. A concept of Inducverters, inspired
by Induction machine working principles has been published
in [20].
Fig. 10. Basic structure of Inducverter
Fig. 9. Phase detector based on second order generalized integrator
B. Non PLL methods:
1) Grid Synchronization using Fourier Analysis: Fourier
analysis is a mathematical tool which transform a given
function from time domain to frequency domain and viceversa. Signal processing techniques can be easily evaluated by
using discrete Fourier transform. By using Fourier series, the
frequency components of a periodic signal can be obtained
by multiplying it by a set of basic sine/cosine functions at
different frequencies. It states that a periodic signal v(t) can
be expressed by a sum of the following terms:
X
v(t) = a0 +
(an cos(nωt) + bn sin(nωt))
(9)
Where the different coefficients are calculated by

RT


a0 = T1 v(t)



0


RT
2
an = T v(t)cos(nωt)dt


0


RT


2

bn = T v(t)sin(nωt)dt
It eliminates the dedicated synchronization process by providing auto start and auto synchronization with the grid without using the grid voltage information, making the controller
unit and synchronization unit integrated into one unit. It
introduces a current damping unit, which enables the autosynchronization by using local current information and it
can thus track grid voltage frequency, angle and amplitude
variation while providing constant power supply. This Inducverter has two main parts: 1) an electrical part, consists
of converter unit, a filter circuit 2) the control part,which by
proper voltage control enables the converter to behave like in
induction machine, and generates pulse width modulated gate
signals to converter unit. The controller has the same features
and characteristics of induction machine such as self and soft
start capability and synchronizing automatically with the grid
without taking any information from the grid.
IV. C ONCLUSION
(10)
0
A selective band pass filter can be implemented by multiplying input signal by the sine/cosine functions at the desired
A panoptical review of different synchronization techniques
particularly for single-phase grid integration has been carried
out to explore a broad perspective on their different features
and applications. In future grid the single-phase grid connection of DGs will have a dominant role and so it has to be more
active and smart. This paper has discussed various singlephase synchronization techniques, which is a major part of
the whole integrated system. Each has advantages as well as
drawbacks. The application of any one of the methods lies on
the precise estimation of amplitude, phase and frequency of
the input signal even under grid disturbances. More detailed
informations may be found in references.
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